REPORTS
ACC.04: Guidelines for Wind Mitigation in Air-Cooled Condensers
Original Issue: 09/10/2024, Revision due: 09/10/2027
Dr. A.G. Howell, Riad Dandan, Hector Moctezuma Guerrero, Dr. R.B. Dooley, Gary Mirsky
Naturally occurring wind has been determined by many researchers, consultants and providers / operators of forced-draft Air-Cooled Condensers (ACCs) as the root cause of performance deficiencies and mechanical problems. Regardless of site-specific wind speed and direction, the equipment is typically designed to meet the maximum wind speed of 5 m/s (16 ft/s) normally specified in test codes, and performance specifications are generally met under such conditions. However, air-cooled equipment must operate during much higher wind speeds and gusts, sometimes frequently. During these periods there can be significant deficiencies in condenser vacuum, in some cases tripping the unit. Wind above 5 m/s will affect thermal performance due to airflow distortion entering the windward fans or recirculation that will impact the windward and leeward fans. Wind can also increase the intake of hot air from neighboring heat sources and can add significant stress to the fan blades by causing loading and unloading as the blades rotate, potentially leading to blade failure. Furthermore, these forces can cause motors overloaded above their amperage set point to trip, and gear reducers can also be damaged due to stresses induced by wind.
Mitigation of the detrimental effects of wind can be achieved to some extent by optimizing the initial design and may include installation or subsequent retrofit of appropriately designed structures. Computational fluid dynamics (CFD) modeling as well as well-designed pilot testing can confirm suitable mitigation efforts.
ACC.03: Guidelines for Air In-Leakage in Air-Cooled Condensers
Original Issue: 09/10/2024, Revision due: 09/10/2024
Dr. A.G. Howell, Riad Dandan, Hector Moctezuma Guerrero, Dr. R.B. Dooley
Steam condensing in thermal power generation creates a vacuum that is important for process efficiency. Air-cooled condensers (ACCs) are subject to inefficiency based on the use of ambient air as the primary agent of cooling rather than water, which is typically cooler than ambient air and therefore produces a better vacuum due to a faster rate of steam condensation. This vacuum is critical to efficient thermal power generation, with a high (stronger) vacuum resulting in a more efficient steam cycle, and therefore a lower rate of fuel consumption. Furthermore, if the vacuum is poor, a reduction in the production of steam may be necessary to protect the steam turbine; consequently, power generation is also reduced. It is routine for thermal power generating plants with ACCs to be required to reduce power output from the steam turbine by 10 to 20% or more during periods of high ambient temperatures.
The vacuum in power plant condensers results in air ingress at any point where the integrity of isolation is compromised. When air enters the vacuum environment of an ACC, routinely installed equipment normally removes the air and no effect on system performance occurs. However, if the rate of air ingress exceeds its rate of removal, air can accumulate in steamside locations and obstruct heat transfer by preventing steam from contacting the surfaces of heat exchange tubing. This effectively reduces the heat exchange surface area and therefore the cooling capacity of the ACC and results in deterioration of the system vacuum. Therefore, it is necessary to monitor the rate of air in-leakage (AIL) and to periodically identify and correct locations of air ingress.
In addition to affecting the vacuum and performance of the plant, oxygen and carbon dioxide present in air can dissolve in condensate and influence steam cycle corrosion and chemistry measurements.
ACC.02: Guidelines for Finned Tube Cleaning in Air-Cooled Condensers
Dr. A.G. Howell, Riad Dandan, Dr. R.B. Dooley, Oscar Hernandez, David Rettke
Original Issue: November 16, 2018. Revision due: November 16, 2021. Publication in whole or in part is allowed provided that attribution is given to the Air Cooled Condenser Users Group.
Steam condensing in air-cooled condensers (ACCs) takes place in specialized heat exchanger tubes designed for optimal heat transfer. Typically, the tubes are composed of carbon steel with exterior (air-side) aluminum coating/cladding, along with external aluminum fins to increase the air-side surface area for heat rejection. Some ACCs, mostly earlier designs, included carbon steel fins on carbon steel heat exchange tubes, with the entire exterior coated with zinc. ACC tubes are normally oval or rectangular in shape and approximately 0.059” (1.50 mm) wall thickness, with closely-spaced fins to facilitate maximum surface area exposure to cooling air flow. Air flow is provided by large fans typically positioned below the array of finned tubes, directing air upward in a so-called “forced draft” situation, although an induced-draft design with fans above the array has been introduced that has some potential advantages. High-purity steam exhausted from the low-pressure steam turbine with a few percent moisture (condensed water) travels through large ducts at relatively high velocity, generally at 120 - 360 fps (35 - 110 mps). Steam velocity is dependent on the vacuum; upon reaching the condenser tubes, flow is redirected and drawn into the tubes with a 90o turn. As the two-phase steam-water mixture moves down the 33 - 36 foot (10 - 11) meter tube length, the remaining steam is condensed to liquid water and releases the latent heat of vaporization.
Flow-Accelerated Corrosion in Steam Generating Plants
Barry Dooley and Derek Lister
Original Issue: September 27, 2018.
Flow-accelerated corrosion (FAC) has been researched for over 50 years at many locations around the world; and scientifically all the major influences are well recognized. However; the application of this science and understanding to fossil; combined-cycle/HRSG and nuclear plants has not been entirely satisfactory. Major failures are still occurring and the locations involved are basically the same as they were in the 1980s and 1990s. This paper reviews the latest theory of the major mechanistic aspects and also provides details on the major locations of FAC in plants; the key identifying surface features of single- and two-phase FAC; the cycle chemistries used in the plants and the key monitoring tools to identify the presence of FAC. The management aspects as well as the inspection; predictive and chemistry approaches to arrest FAC are described; and the different approaches that are needed within fossil; HRSG and nuclear plants are delineated.
ACC.01: Guidelines for Internal Inspection of Air-Cooled Condensers
Dr. A. G. Howell, Gary Bishop, David Rettke, Rene Villafuente, Dr. R.B. Dooley, Hoc Phung
Original Issue: May 12, 2015. Revision due: May 12, 2018. Publication in whole or in part is allowed provided that attribution is given to the Air Cooled Condenser Users Group.
The employment of air-cooled condensers (ACCs) in fossil fueled power-generating facilities has increased dramatically early in the 21st century. These condensers are massive structures at power plant sites, because a large cooling surface is required to compensate for the relatively poor heat capacity of air, in comparison with that of water. As experience has been gained in operating and maintaining facilities with ACCs, it has become clear that corrosion of steam-side surfaces can be a significant problem for unit operation. In particular, iron oxide transport can introduce a large quantity of contaminants to the condensate / boiler feedwater, and through-wall penetrations of cooling tubes can cause significant air inleakage, potentially leading to reduced condenser performance and permitting ingress of air.
Corrosion in Air Cooled Condensers - Understanding and Mitigating the Mechanisms
